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CEL P0421 Help - '99 LX

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    CEL P0421 Help - '99 LX

    Roughly 2 months ago I got a CEL which lead me to replace the downstream 02 sensor. I'm searching my desk, but can't find the original code. I believe it related to a voltage issue.

    After replacing it and clearing it, everything was fine until this afternoon when the light came back. Got the code read and it's a P0421.

    After reading around, I'm not sure where this might be pointing. The text I've found is: P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1) .
    This isn't pointing to the upstream, I've read some posts that this could be an indication the cat is going bad or already has.

    This is a '99 LX, stock everything with about 115k on it.

    Any help would be appreciated. Thanks guys.

    #2
    Yep, its the cat-pretty common on those(and lot of the 2nd gens too)

    Comment


      #3
      Yeh its a good chance it is the cat.....however, also you could have a ridiculously big vacuum leak somewhere....or like you said a voltage issue....let's hope its not the latter of the 3 choices orelse your gonna need a good multimeter and alot of patience to track down each line.....If it is a voltage issue however, PM me I think I have a pdf file where you can track down the wires through the wire harness and which wires to test.

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        #4
        The average multimeter can't test an O2 sensor. If you try to measure resistance you can actually damage it by sending too much voltage across it. The sensor is probably bad or you may have replaced the wrong one. A few things that I've found that helps: Clean the throttle body, clean mass air flow sensor, replace air filter, clean the pipe that comes off the intake manifold and goes out to the EGR valve. I have 220k miles and the original cat.

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          #5
          to test an O2 sensor you put the scale in the minimum Voltage scale, not OHMs.
          sigpic My cardomain

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            #6
            Testing the heater can be done with an ohm meter, but it's not safe to do so because it's easy to burn it out. Testing the signal voltage doesn't serve much purpose because it fluctuates (cycles) and your average multimeter isn't sensitive (fast) enough to test the difference between the peaks.

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              #7
              Time for a header and nonfouler.
              AEM intake, Ractive exhaust, Tsudo header, Ecoated blanks, and a bunch of stickers

              Comment


                #8
                Originally posted by diehonda View Post
                Time for a header and nonfouler.:
                Where do i get a nonfouler please?
                Check out my 4WD Compact Car thread.

                Comment


                  #9
                  dtatham is correct - you cannot measure the output voltage of an O2 sensor with a VOM. You need an OBD scanner which is capable of displaying O2 sensor outputs (or an oscilloscope).

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